Decide
The Lisbon table shows a low water at 10:25 on a day in August. What is the legal time in mainland Portugal?
The lesson in plain words
- Who publishes it
- The Instituto Hidrográfico (IH) of the Portuguese Navy publishes the Tabela de Marés every year.
- Volume I covers ports in mainland Portugal, the Azores and Madeira; it is now published in digital form.
- Each main port has monthly pages with the predicted preia-mar (PM, high water) and baixa-mar (BM, low water).
- One day, four lines
- Most days show four lines: two high waters and two low waters, each with a time and a height.
- Some days have only three, because the tide runs about 50 minutes later each day and one event slips past midnight.
- Read the day number and weekday on the left, then each time with its height.
- Times are UTC
- The page header says Horas do Fuso: 0 (TU): the predictions are in UTC.
- Mainland Portugal and Madeira use UTC in winter and UTC+1 in summer, so in summer you add one hour.
- The Azores are one hour behind the mainland.
- The IH warns that the table's time zone may differ from legal time.
- Heights above chart datum
- Heights are in metres above the zero hidrográfico (ZH), the same datum as the soundings on Portuguese charts.
- On Portuguese charts the ZH lies below the lowest astronomical tide, so the predicted heights are always positive.
- A low water of 0.8 m means 0.8 m of water above the charted sounding, not 0.8 m of depth.
Key data
- Publisher
- Instituto Hidrográfico, Tabela de Marés, Volume I
- PM / BM
- Preia-mar (high water) / baixa-mar (low water)
- Time zone
- Fuso 0 (UTC); mainland summer legal time is UTC+1
- Height datum
- Zero hidrográfico (ZH), the chart datum
- Lisbon mean level
- 2.20 m above ZH (IH 2026)
Key points
- Each line is one high or low water: time and height.
- Times are UTC: add one hour for mainland summer time.
- Heights are metres above the zero hidrográfico, never depths.
- Always use the current year's table for the right port.
Check what you have read
The same table shows a high water at 13:19 on a day in January. What is the legal time?
True or false?
Decide each statement before you see the answer.
Each line is one high or low water: time and height.
Times are UTC: add one hour for mainland summer time.
Heights are metres above the zero hidrográfico, never depths.
The times in the table are already in Portuguese legal time.
Last year's table will do, the tides repeat.
A low water of 0.8 m means there is 0.8 m of water where I am.
Practice case & exam keys
Situation
On Saturday 1 August 2026 the IH table for Lisbon shows: 04:41 3.4 m, 10:25 0.8 m, 16:54 3.6 m, 22:52 0.7 m. You plan to leave in the afternoon. At what legal time is the afternoon high water, and how high is it?
Show the answer
16:54 is in UTC. On 1 August mainland Portugal is on summer time (UTC+1), so high water is at 17:54 legal time, with 3.6 m above chart datum. The morning low water was at 11:25 legal time with 0.8 m. Read the 16:54 line as a high water because it is higher than the low waters on either side.
Exam keys
- PM = preia-mar (high water); BM = baixa-mar (low water).
- IH tables: times in UTC (Fuso 0); add 1 h in mainland summer time.
- Heights in metres above the zero hidrográfico (chart datum).